fix(google): deliver tool calls before combined turn completion

[noooooooookro]

Fixes #7151

What changed

Google Realtime can send a single response containing both terminal server content and a tool call. The receive loop now defers generation finalization for that server content, keeping the function channel open until the tool call is delivered. Existing responses without a tool call retain their current completion behavior.

Validation

• python -m pytest tests/test_plugin_google_realtime.py -q (19 passed)
• ruff check livekit-plugins/livekit-plugins-google/livekit/plugins/google/realtime/realtime_api.py tests/test_plugin_google_realtime.py
• ruff format --check livekit-plugins/livekit-plugins-google/livekit/plugins/google/realtime/realtime_api.py tests/test_plugin_google_realtime.py

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[longcw]

The behaviour is right. Maybe a cleaner shape: finalize here instead of in the two handlers. Both _handle_server_content and _handle_tool_calls are only called from this loop, so the loop can own the decision and the defer_completion flag goes away.

if response.server_content:
    self._handle_server_content(response.server_content)
if response.tool_call:
    self._handle_tool_calls(response.tool_call)
if ((sc := response.server_content) and sc.turn_complete) or response.tool_call:
    self._mark_current_generation_done()

Then drop the turn_complete finalize at the end of _handle_server_content and the one at the end of _handle_tool_calls. Same order as now: content, then the calls, then close, in one iteration.